I understand how important it is to print your checks on time in your QuickBooks Desktop (QBDT) Mac, @AVIDPgh. Moreover, I appreciate your taking the time to raise this concern on our forum. I'm here to share some information that can help you fix this.

This issue needs further isolation and checking of your account promptly. Since we're unable to access your account here, I recommend contacting the QBDT Mac Live Support Team. Their tools are equipped to intricately review your printing settings and determine the root cause of why you can't achieve your goal on your checks. Here's how:

1. Open your QBDT Mac and hover your cursor to the Help tab.
2. Pick QuickBooks Desktop Help/Contact Us.
3. Type in any keyword that relates to your printing checks issue.
4. Select Contact Us to be connected to expert help.

The available hours for Mac support are Monday - Friday 6 AM to 6 PM.

In the meantime, you can utilize the system's suggestion about using a US letter paper size printing so you can finish your task. 

Let me know how it goes, @AVIDPgh. Or, if you have additional questions about managing and printing checks, leave a comment below. I got you always. Have a good day.

Hello @AVIDPgh 

On the Print window check try the following to see if this will help.

Leave the Paper Size set to US Letter

Expand the QuickBooks section

On the Options tab choose Standard

Checks on first Page, choose One

Partial Page choose the option that works best you (this will allow the input to be similar to the envelope option).

This should allow you to print out a single check.

My HP printer then tells me I have a paper size mismatch and won't print

Is it setup to something similar to this?  

I'm having the same issue. None of the suggestions above have lead to a resolution.

I appreciate your effort in trying the provided resolution above, @Khoopy.
 

I suggest contacting our Live Support team directly to review your print settings and determine the cause of the checks not printing. Here's how to reach them.
 

1. Navigate to the Help icon on the upper right corner of your screen.
2. Choose the Contact Us option. 
3. Select Contact Us below.
4. Input the details about your concern and click Continue.
    

Also, to learn our support availability hours, you can visit this material: Get help with QuickBooks products and services.
 

On top of that, to maintain positive vendor relationships by ensuring that payments are never late and to save time managing regular payments, you can read this article: Create an Automatic Recurring Payment (Check) to a Vendor.
 

Please let us know in the comments below if you have any additional questions or concerns regarding checks in QuickBooks. We're here to help.

Hello @AVIDPgh  and @Khoopy , a new update was just released (R9.2) that has the ability to print a single check using the envelope feeder.  When printing, if you choose the option for one check on 1st page, it will allow you to choose custom paper sizing options now.  Hope this helps.